1. Field of the Invention
The present invention relates to supporting glass sheets and, more particularly, to improved means for applying a rotational force to rotate frangible ceramic rolls in a roller hearth conveyor within a heated furnace. The present invention also relates to using such rolls to convey glass sheets during thermal treatment.
It has been customary to use rotating metal rolls to convey glass sheets and other materials through tunnel-like furnaces for heat treatment. Recently, more frangible ceramic rolls have been used because they do not mar the glass sheets that are conveyed by frictional rotation thereover through a heat treatment zone where the glass sheets are initially heated to a temperature sufficient for further processing and then cooled during their further processing.
A typical heat treatment operation involves tempering glass sheets. When glass sheets are tempered, they are heated to an elevated temperature considerably higher than the strain point of the glass. The hot glass is then chilled rapidly to impart a stress pattern throughout the thickness of the glass. The stress pattern is such that the surface portions of the glass are stressed in compression and the interior of the glass within the surface portions is stressed in tension.
Glass sheets that develop a stress distribution comprising a skin of compression stress surrounding an interior stressed in tension become tempered to produce a glass product much stronger than untempered glass. Tempered glass is less likely to shatter than untempered glass when struck by an object. Furthermore, in the less frequent times when an outside force is sufficiently large to cause tempered glass to fracture, tempered glass breaks up into a large number of relatively smoothly surfaced, relatively small particles which are far less dangerous than the relatively large pieces with relatively jagged edges that result from the more frequent fracture of untempered glass.
To promote efficient and large scale production, discrete glass sheets are conventionally heated and cooled while being moved continuously along a fixed path and successively through a heating section, a quenching section and a cooling section. The path is defined by a common upper tangent of rotating conveyor rolls, each of which is longitudinally spaced from one another along the length of the path and each of which extends transversely across said fixed path.
To achieve satisfactory temper, the temperature of the glass sheet must be above a predetermined minimum level so as to maintain the core or interior thereof above a deformation temperature upon being exposed initially to the quenching medium of the quenching section. The residual heat remaining in glass sheets should be sufficient for immediate advancement to the tempering area and exposure to the quenching medium.
Glass sheets that are rotated on relatively fragile ceramic conveyor rolls during tempering are less likely to develop a defect known as roll ripple distortion than glass sheets that are conveyed on stainless steel rolls. However, since roll ripple distortion is associated primarily with a critical portion of the apparatus adjacent the exit of the furnace and in the entrance to the further treatment zone wherein the glass sheets are at their maximum temperature and highly susceptible to roll ripple distortion, it has been found practical to limit the substitution of the relatively expensive ceramic rolls for the less expensive metal rolls of the prior art to the aforementioned critical portion of the conveyor.
Ceramic conveyor rolls in high temperature regions are preferable to metal rolls for several reasons. One of these reasons is that the ceramic rolls expand less thermally. Metal rolls tend to sag or buckle at elevated temperatures and fail to provide a straight line of support in the tangential plane common to the upper portions of the circumferences of the rotating rolls. The tendency of ceramic rolls to be brittle and fracture more readily than metal rolls has caused the glass sheet heat treatment industry to search for a combination roll for conveyors usable in high temperature furnaces that combine the best characteristics of ceramic rolls with the best characteristics of metal rolls.
2. Description of the Prior Art
U.S. Pat. No. 3,338,569 to Cuvelier discloses a ceramic roll drive system for a tunnel-like furnace. The system incorporates a series of ceramic rolls arranged side-by-side for conveying materials through the furnace. Spindles extend into the tunnel from opposite sides and project into the opposing ends of the respective ceramic rolls to support the latter. A suitable drive system is provided to rotate the spindles. The spindles actually extend into axially disposed holes provided in the opposing ends of the ceramic rolls. Presumably, there is a slight interfit therebetween to permit the driven spindle to positively rotate the ceramic roll.
The ceramic roll of the conveyor is hollowed out to telescopically receive the inner ends of a driven spindle. The length of the hollowed out enlarged end portion is sufficient to permit some longitudinal expansion in an axial direction of the spindle or the ceramic roll forming part of the furnace.
U.S. Pat. No. 3,489,397 to Alexander discloses a roller hearth furnace in which the individual rolls of the roller hearth conveyor are driven by respective sprockets. Each sprocket is drivingly connected to its associated roll via a U-shaped spring arrangement that provides a friction drive. Each ceramic roll is received at each end in a sleeve 17 that is journaled for rotation in a bearing.
U.S. Pat. No. 3,608,876 to Leaich discloses a system of coupling ceramic rollers to driving and supporting shafts by uniquely arranged pin and slot connections which coact with the supporting shafts to effect a positive rotational drive to each of the conveyor rolls.
U.S. Pat. No. 3,867,748 to Miller discloses a ceramic roll drive system for use in a heating furnace that includes metal end caps positioned on the opposing ends of the ceramic roll. Each end cap is provided with an annular recess that receives a supply of a heat expandable adhesive material. The adhesive material thus contacts and engages with peripheral portions of the opposing ends of the roll. This reference also discloses a compression spring arrangement for applying axial forces to the roll and its mounting assembly including the end caps. The arrangement maintains the conveyor rolls, the hubs, the end caps and ceramic tube of each roller assembly pressed together in the desired relationship. Presumably, this arrangement also serves to take up and allow for thermal expansion.
None of the aforesaid references take advantage of the fact that ceramic material used in roller hearth conveyor rolls is exceedingly strong in compression so as to provide a combination of a compressive force and friction facings between metal supporting elements and a ceramic roll in the axial direction of the roll. The adhesive provided in the Miller patent is applied between adjacent circumferential surfaces. This tends to develop a tortional stress in tension which tends to cause fracture of the ceramic rolls.
U.S. Pat. No. 4,057,411 to Reese discloses a roller hearth conveyor that has rolls only in a critical portion of the conveyor maintained in more precise alignment to reduce roll ripple distortion to a considerable extent.
Other references reported in a novelty search of the present invention include U.S. Pat. Nos. 2,531,984 to Millar; 2,671,262 to Kuniholm; 2,890,517 to Mengel; 3,111,823 to Heinz Kater; 3,879,786 to Larkin; 3,881,234 to Crowell et al; 3,994,380 to Hope et al; and 4,034,837 to Vinarcsik et al. None of these additional references are believed to be as pertinent as those which were discussed earlier.